TCP push packet not delivered from tun

Question

I setup simple packet intercept program, using two tuns, setup like this:

# ip tuntap add mode tun name tun0
# ip link set tun0 up
# ip addr add 10.0.0.0/31 dev tun0
# ip tuntap add mode tun name tun1
# ip link set tun1 up
# ip addr add 10.0.1.0/31 dev tun1

and redirect output to the program like this:

# ip rule add fwmark 1 table 1
# ip route add default dev tun0 table 1
# iptables -t mangle -A OUTPUT --source 192.168.1.0 -o enp34s0 -p tcp --dport 9732 -j MARK --set-mark 1
# iptables -t nat -A POSTROUTING --source 10.0.1.1 -o enp34s0 -j MASQUERADE

I enabled ip_forward and disabled rp_filter. Packets received on tun0 are processed, modified and ip/tcp checksums are updated. I can even correctly intercept tcp handshake SYN -> ACK,SYN -> ACK part of communication, but after that, any incoming packet would be correctly intercepted modified and send out of tun, but it would never be delivered to local application.

Answer 1

Ok, found a problem, whilst I did recalculate the checksum, it only calculated correct one only for the TCP packets without any payload, thus TCP handshak get through and nothing else.